1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display (LCD) apparatus, and more particularly, an improvement of the drain/source electrodes of a thin film transistor (TFT).
2. Description of the Related Art
Active matrix-type LCD apparatuses are so thin and light that they are often used in various display apparatuses. Also, since the active matrix-type LCD apparatuses can be of a large size and fine screen with a high quality and a low power dissipation, cathode ray tubes (CRTs) have been replaced with the active matrix-type LCD apparatuses.
In an active matrix-type LCD apparatus, one TFT is provided as a switching element for each pixel. The TFT is constructed by a gate electrode, a semiconductor active layer made of amorphous silicon opposing the gate electrode, a source electrode, and a drain electrode connected to a transparent pixel electrode. Also, the gate electrode is connected to one gate bus line, and the source electrode is connected to one source bus line.
In a first prior art LCD apparatus, the source bus line and the transparent pixel electrode are formed on the same surface of a gate insulating layer. This will be explained later in detail.
In the first prior art LCD apparatus, however, since the space between the source bus line and the transparent pixel electrode needs to be 10 .mu.m or more, in order to avoid a short-circuit, the numerical aperture is reduced.
In a second prior art LCD apparatus (see JP-A-2-208636), the source bus line is formed on the gate insulating layer, while the transparent pixel electrode is formed on another insulating layer. In this case, the transparent pixel electrode is connected via a contact hole perforated in the insulating layer to the drain electrode. Therefore, since the source bus line is electrically isolated from the transparent pixel electrode by the insulating layer, the space between the source bus line and the transparent pixel electrode can be reduced to thereby increase the numerical aperture. This will be also explained later in detail.
In the second prior art LCD apparatus, however, a resistance between the transparent pixel electrode and the drain electrode becomes unstable which deteriorates the characteristics of the TFT. A dry etching process as well as the formation of a photoresist pattern for forming the contact hole invites the contamination of the surface of the drain electrode. Further, if the transparent pixel electrode is formed by a sputtering process, the surface of the drain electrode is also in an oxidation state. In addition, the surface of the drain electrode is oxidized by oxygen included in the transparent pixel electrode.
In a third prior art LCD apparatus (see JP-A-5-188397), in order to obtain good coverage characteristics of the transparent pixel electrode to the drain electrode without a dry etching process, a wet etching process is carried out twice to form a contact hole in the insulating layer between the transparent pixel electrode and the drain electrode.
The double wet etching processes prevent the surface of the drain electrode from being contaminated, so that the resistance between the transparent pixel electrode and the drain electrode becomes stable. This will be explained later in detail.
In the third prior art LCD apparatus, however, it is impossible to completely avoid the disconnection of the transparent pixel electrode around the edges of the contact hole. In addition, the double wet etching processes associated with two photolithography processes increase the manufacturing cost and reduce the manufacturing yield.
In a fourth prior art LCD apparatus, after the above-mentioned contact hole is formed, a sputter-etching process is performed upon the drain electrode to remove the contamination of the surface thereof. Then, the transparent pixel electrode is formed.
In the fourth prior art LCD apparatus, however, even during a step for depositing the transparent pixel electrode, the surface of the drain electrode is oxidized. In addition, a large scale sputter-etching apparatus is required which increases the manufacturing cost.